Mobile communications devices have become an integral part of society over the last two decades. Indeed, more than eighty-two percent of Americans own a mobile communications device, for example, a cell phone. The typical mobile communications device includes an antenna, and a transceiver coupled to the antenna. The transceiver and the antenna cooperate to transmit and receive communications signals.
The typical transceiver includes a power amplifier for amplifying low voltage signals for transmission via the antenna. Given that most mobile communications devices operate on limited battery power, energy efficient power amplifiers may be desirable. More specifically and as will be appreciated by those skilled in the art, Class C and E power amplifiers are common in mobile communications devices since they are efficient power amplifiers. These classes of power amplifiers are more efficient than Class A or B amplifiers, for example, but are subject to performance tradeoffs. For example, they may be non-linear over certain frequencies and may introduce greater amounts of distortion into the amplified signal (if the signal requires a linear amplifier).
In some mobile communications applications, it may be helpful to implement orthogonal frequency-division multiplexing (OFDM), which has a high spectral efficiency and robustness in multipath environments. Nonetheless, since OFDM signals have a high peak-to-average power ratio (PAPR), the use of non-linear and efficient Class C or E amplifiers may introduce unwanted distortion into the OFDM signal. Accordingly, the typical OFDM mobile communications device may include less efficient yet more linear amplifiers, thereby undesirably decreasing battery life.
An approach to compensating for this tradeoff is to encode transmitted signals with constant envelope (CE) or continuous phase modulation (CPM) waveforms. These modulations provide for lower energy demands on the power amplifier of the transceiver, for example, by reducing the PAPR, increasing average transmit power (providing greater transmission range), and increasing amplifier efficiency.
For example, U.S. Patent Application Publication No. 2008/0039024 to Ikeda et al. discloses an amplifying circuit. The amplifying circuit processes an input orthogonal frequency-division multiplexing (OFDM) signal and provides a pair of CE output signals to provide better power efficiency and less distortion. A particularly advantageous approach is constant envelope orthogonal frequency-division multiplexing (CE-OFDM).
The CE version of OFDM does not typically use pilot tones to provide channel estimation. Hence, the typical CE-OFDM mobile communications device may include a frequency domain equalizer (FDE). A potential drawback to the typical CE-OFDM mobile communications device is the undesired introduction of distortion into the received signal by the FDE and other components.
An approach to address this drawback is disclosed in U.S. Patent Application Publication No. 2006/0274641 to Grieco et al. This CE-OFDM device includes two-stage equalization of a received CE-OFDM signal. More specifically, the CE-OFDM device includes iterative demodulation between pre and post demodulation equalizers.